1. Field of the Invention
The present invention is related to a converter which converts an analog signal to a digital signal (an analog-digital converter), and a converter which converts a digital signal to an analog signal (a digital-analog converter). In particular, the present invention is related to an analog-digital converter and a digital-analog converter, each of which uses a ΔΣ modulator.
2. Description of the Related Art
As a method for realizing a high accuracy analog-digital converter and a high accuracy digital-analog converter, a method to use a ΔΣ modulator as shown in FIG. 1 for example is used. In such a method to use a ΔΣ modulator, quantization of an input signal passed through a loop filter is once performed at a lower resolution than the precision finally required and the result is fed back to the input. In addition, over-sampling is performed at a sampling frequency higher than the sampling frequency finally required. By performing the feedback process described above at a high sampling frequency which is for over-sampling, the frequency distribution of quantization noise caused by quantization of the low resolution is controlled and noise in the signal band is reduced. This method is called noise shaping. Even in the case where a low resolution quantizer is used, it is possible to obtain high conversion accuracy by noise shaping. Noise within the signal band can be reduced by increasing a ratio (an oversampling ratio) of the sampling frequency finally required and the sampling frequency by performing over-sampling.
Therefore, in the case of obtaining high conversion accuracy or a high signal-to-noise ratio (SNR), it is necessary to increase the oversampling ratio. For example, it is necessary to increase sampling frequency of the oversampling by 100 times the output sampling frequency.
As a result, however, since the clock frequency is increased, the clock frequency component, the high range noise component which has been noise shaped, and the harmonic component of the signal which has been further sampled (image signal) are radiated as an electromagnetic wave, and a problem arises whereby unnecessary radiation is transmitted to other circuits via wires such as a power supply and adversely affects other circuits and equipment.
As a means for solving this problem, there is a method to apply modulation to a clock signal of an analog-digital converter or a digital-analog converter as shown in FIG. 2.
In the case where this method is used in a converter such as a ΔΣ modulator, however, a problem arises whereby conversion accuracy is significantly degraded. That is, in the case where a clock frequency is varied, low frequency noise is greatly increased and the conversion accuracy is deteriorated.